Will it be possible to employ a discrete Fanless GPU, for example, Sapphire ATI HD7750 Ultimate or AMD Radeon™ HD 6850, with this case?

Passive GPU's rely on airflow within the case to cool them. This case uses itself as a huge heatsink and relies on the minor airflow in the room to keep the processor cool. If you put a GPU inside of this case it will probably overheat because there is absolutely no airflow.